BTU (British Thermal Unit) or energy-content monitors are used to analyze the BTU content of natural gas at producing wells and transfer points, and also at the points of consumption. Previous work has showed the possibility of accurately performing this analysis by near infrared (NIR) spectroscopy, but commercially available instrumentation was too complex and expensive to be deployed widely. Instead gas chromatographs (GC) are currently used widely to perform this BTU monitoring. Nonetheless, there are several advantages to using NIR for this analysis including lower total cost of ownership, lower maintenance, faster response (seconds instead of minutes or tens of minutes), and the NIR approach requires no consumables such as carrier and fuel gases required by the GC.
In a GC analysis, the hydrocarbons present in the natural gas are separated based on the retention time on a heated column. The BTU or energy content is calculated by summing up the concentration of the different hydrocarbon species and applying published mathematical equations.
NIR based analysis operates on a different principle. A spectrum of the sample is collected. Light at different wavelengths is absorbed based on the concentration of functional groups, such as C—H, C═C—H, C═C—H, N—H, and O—H, present in the molecules of the gas. The spectrum of the natural gas is a sum of the different species present such as methane, ethane, isobutene, n-butane, propane, etc.
Mathematical models based on chemometrics derive a relationship between the spectra and the concentration of the property of interest, such as BTU or energy content. The resulting calibration model is applied to each spectrum from the gas, and the BTU content can be reported to a local control or data logging system.